Mr Champion said the bill was "too important to water down or get wrong."
He was referring to a dilution of the bill by National MPs which would allow businesses with fewer than 20 employees to refuse to have a health and safety representative upon request of an employee. At present, and in the original bill, a health and safety rep had to be elected if an employee asked for one.
Mr Champion said the idea of a threshold for compliance was "deeply flawed."
He asked why a business of 27 largely clerical-based employees should have to comply with the law and a trenching company of, say, 12 staff not have to comply even though trenching was clearly a more dangerous activity.
"To require these prescriptive regimes is counterproductive and counter-intuitive."
Mr Champion said the workplace enforcement regime needed to be well-resourced and active.
The EMA welcomed the addition of 60 workplace safety inspectors but it would take another 30 to meet ILO [International Labour Organisation] recommended ratios.
The week that the bill was due to be reported back from the transport and industrial relations committee, the Government announced it would delay the report-back by a few months. Backbenchers were said to have voiced concerns at caucus about the effects on small business.
Prime Minister John Key said he wanted more time to get it right but that was about a month ago.
Mr Champion's letter is a departure from the EMA's original position on the Government's decision to delay the report of the bill from select committee by a couple of months.
At that time, chief executive Kim Campbell said the EMA supported the delay if it mean getting the right result for employers and workers.
"Let's have good law, not quick law because health and safety is too important to get wrong," Mr Campbell said.
Mr Champion's letter was the subject of questions to Workplace Relations and Safety Minister Michael Woodhouse.
Mr Lees-Galloway, Labour's industrial relations spokesman , said later that the letter from the EMA, which represents thousands of small businesses, was "a slap in the face for the Government."
Unions had all voiced their concerns about the direction the bill was going in.
"Now employers are speaking up about the Government's woeful management of the reform process too.
"No one has any idea what further changes the Government intends to make to the bill and that's leaving everyone very nervous about what happens next."
One News reported tonight that Justice Minister Amy Adams has asked Mr Woodhouse to include a new measure in the bill to allow charges of corporate manslaughter although that is not thought to be the reason the legislation was delayed.